Single-crystal alumina (sapphire), which is transparent and excellent in heat resistance, wear resistance and corrosion resistance, has an excellent property in which it can be used even in a severer environment where metallic material or organic material is not usable. However, the single-crystal alumina can be formed into only a material of simple shape such as sheet or bar, since a process for melting alumina at a high temperature of a melting point (2050° C.) or higher in a crucible and doping and pulling a seed crystal to thereby grow the crystal (CZ process) or for depositing alumina powder in a melt state over a seed crystal to thereby grow the crystal (Verneuil Process) is adapted for production of the single-crystal alumina.
Further, sapphire is limited in available areas, since it is basically a hard and brittle material, and thus difficult to machine from the material.
On the other hand, polycrystalline alumina (PCA) is extensively used as a sintered body almost free from residual pores by baking a compact composed of alumina fine powder at a temperature lower than the melting point. Since the alumina fine powder can be shaped by use of various molding methods with high shape flexibility, alumina sintered bodies in various shapes are produced and industrially used.
Although the polycrystalline alumina was limited in uses to simple wear resisting and heat resisting members since it was basically impenetrable to light, Coble of US succeeded in development of a translucent polycrystalline alumina sintered body by sintering a high-purity alumina raw material with minimized impurities while adding a grain growth inhibitor, to allow the use to a luminous vessel for general lighting high-pressure sodium lamp or metal halide lamp (U.S. Pat. No. 3,026,210).
If a transparent alumina material further improved in translucency can be developed, improvement in luminous efficiency by reduction in loss of light by scattering and extension of the usable range not only as general lighting but also as point light source can be attained. From this point of view, in Japanese Patent Publication No. 07-165485A, a method for attaining both shape flexibility and transparency of polycrystalline alumina by converting a polycrystalline alumina sintered body to a single crystal body by contact with single-crystal alumina to thereby form a transparent body is proposed.
In Japanese Patent Publication No. 2001-519969A and Japanese Patent Publication No. 2003-157798A, it is proposed to produce a metal halide luminous vessel by joining a polycrystalline alumina sintered body to a single-crystal alumina vessel.
In Japanese Patent Publication No. H2-64603A, an invention of shrink-fitting a sapphire disk to the inside of a polycrystalline alumina vessel to be used as an observation window is disclosed.
In the method of Japanese Patent Publication No. H07-165485A, it is difficult to control the growing direction of crystals in the whole member to an optional direction, although the polycrystalline alumina can be partially converted to single crystals, and this method is hardly applicable to a complicated shape.
In the metal halide luminous vessel with polycrystalline alumina members shrink-fitted to both ends of a sapphire vessel, which is proposed in Japanese Patent Publication No. 2001-519969A and Japanese Patent Publication No. 2003-157798A, the sapphire vessel is difficult to produce and also high in cost, and straight traveling of light is disturbed by surface irregularities on the vessel surface characteristic to a crystal growing plane caused during crystal growth. Therefore, machining may be needed to smoothly finish the irregular surface, and in such case, the cost is further increased.
A light guide member including the sapphire disk shrink-fitted to the polycrystalline alumina vessel, which is disclosed in Japanese Patent Publication No. H2-64603A, is used for observing the internal state through the transparent sapphire window, and not aimed at application to a high-luminance discharge lamp luminous vessel, and no technical disclosure for developing airtightness is shown therein.